[clang][OpenMP] Emit lastprivate final copies in no-loop kernels
Promotion rejected lastprivate because the no-loop branch leaves the
worksharing path before it privatizes or copies out, so the clause would
have been silently dropped.
Privatize the non-counter variables in the parallel region and copy them
out under the last iteration that applyWorkshareLoop publishes, forcing
the exit barrier the copy reads through. Loop counters stay at the
distribute level and reach EmitOMPSimdFinal.